The present invention relates to adaptive error concealment, and more particularly, to such concealment for use with digital television tape recording.
When reproducing recorded sample signals, a loss of signal ("dropout" sometimes occurs due to scratches or other imperfections on the surface of the magnetic tape, etc. These dropouts are usually of sufficient duration to cause the loss of many recorded samples, thus producing a burst error. To conceal such dropouts, it is known to use time spreading schemes such as shown in U.S. patent application Ser. No. 139,237 filed Apr. 11, 1980, in the names of J. K. Clemens and N. R. Corby, Jr. and U.S. patent application Ser. No. 170,811 filed July 21, 1980 in the name of G. A. Reitmeier. In these schemes, adjacent samples of the original video signal are non-adjacently recorded on the tape by using multiple tracks and/or time delay elements. Upon reproduction, the original sample order is reobtained using further time delay elements. Thus when a dropout occurs, it affects samples that are adjacent on the tape, but non-adjacent in the reproduced video signal, i.e., it is "spread out", as shown in FIG. 1, wherein "X" and "O" represent dropped-out and good samples respectively. Since only one of every four samples is incorrect in these particular arrangements, the good samples surrounding an erroneous sample can be used to generate an estimate to replace the erroneous sample, thereby concealing the error.
The simplest method for error concealment is to simply average together the two closes samples to the sample in error which have identical subcarrier phase, which samples are shown in FIG. 2. Since with three times subcarrier frequency sampling, every third sample has the same subcarrier phase, the estimated value f(n) for a dropped-out sample p(n) is: EQU fn=1/2p(n-3),
where p(n-3) is a sample taken three sample periods before sample n, and p(n+3) is a sample taken three sample periods after sample n. This method may not correct perfectly in areas of color luminance transitions, since it causes smearing.
In the aforementioned Reitmeier application, an adaptive method of error concealment was described for a signal sampled at a rate of four times the subcarrier frequency. Because of the reduced data rate of three times subcarrier sampled video, it may be desirable to use this rate for digital tape recording in spite of the decreased horizontal resolution, particularly where portability and power considerations are important since the lower data rate causes less tape and power usage. The aforementioned Reitmeier approach to error concealment formed estimates of the sample in error from different spatial directions, e.g. horizontal and vertical, and adaptively chose one of the said estimates based on a criterion of minimum direction of change. In the case of a three times subcarrier frequency sampling, the resulting non-orthogonal sampling pattern makes the previous approach more difficult. In any case, the use of such an adaptive method requires the use of at least 2H (120 microseconds for NTSC) of digital delay, which is undesirable in those applications where portability, power consumption and cost are of utmost concern, and where a sampling rate of three times subcarrier frequency sampling might be used.
It is therefore an object of the present invention to provide improved error concealment for digital video signals, particularly signals sampled at three times the color subcarrier frequency which signals are thereafter recorded.